A human auditory system includes an outer ear, middle ear, and inner ear. A sound source such as a loudspeaker in a room may output sound. A pinna of the outer ear receives the sound, directs the sound to an ear canal of the outer ear, which in turn directs the sound to the middle ear. The middle ear transfers the sound from the outer ear into fluids of the inner ear for conversion into nerve impulses. A brain then interprets the nerve impulses to hear the sound. Further, the human auditory system perceives a direction where the sound is coming from. The perception of direction of the sound source is based on interactions with human anatomy. This interaction, includes sound reflecting, scattering and/or diffracting with the outer ear, head, shoulders, and torso to generate audio cues decoded by the brain to perceive the direction where the sound is coming from.
It is now becoming more common to listen to sounds wearing personalized audio delivery devices such as headphones, hearables, earbuds, speakers, or hearing aids. The personalized audio delivery devices outputs sound, e.g., music, into the ear canal of the outer ear. For example, a user wears an earcup seated on the pinna which outputs the sound into the ear canal. Alternatively, a bone conduction headset vibrates middle ear bones to conduct the sound to the human auditory system. The personalized audio delivery devices accurately reproduce sound. But unlike sound from a sound source, the sound from the personalized audio delivery devices does not interact with the human anatomy such that direction where the sound is coming from is accurately perceptible. The seating of the earcup on the pinna prevents the sound from interacting with the pinna and the bone conduction may bypass the pinna altogether. Audio cues are not generated and as a result the user is not able to perceive the direction where the sound is coming from.
To spatialize and externalize the sound while wearing the personalized audio delivery device, the audio cues can be artificially generated by a head related transfer function (HRTF). The HRTF is a transfer function which describes the audio cues for spatializing the sound in a certain location for a user. For example, the HRTF describes a ratio of sound pressure level at the ear canal to the sound pressure level at the head that facilitates the spatialization. In this regard, the HRTF is applied to sound output by the personal audio delivery device to spatialize the sound output in the certain location even though the sound does not interact with the human anatomy. HRTFs are unique to a user because the human anatomy between people differ. The HRTF which spatializes sound in one location for one user will spatialize and externalize sound in another location for another user.
The drawings are for the purpose of illustrating example embodiments, but it is understood that the embodiments are not limited to the arrangements and instrumentality shown in the drawings.